This invention relates in general to fasteners and, more specifically, to nut, washer and bolt combinations used to secure overlapping metal plates and the like together.
In the past, large metal structures such as steel water tanks or other large containers for powdered or granular material or corrosive liquids and gasses have been assembled from component parts and secured together in a variety of ways, including welding, rivet and bolts. While all of these are effective, welding and riveting on site require considerable equipment and highly skilled assemblers. Fastening with metal bolt, washer and nut combinations, using factory pre-drilled holes in overlapping plates, is often the fastening technique of choice. Generally, conventional steel bolts, nuts and washers are used, with in some cases a plastic cap placed over the nut.
For greater resistance to corrosion and resulting increased durability, improved appearance and reduced contamination of the contained material, glass enamel coated steel plates have come into increasing use. The use of the usual steel nuts, washers and bolts with these new coated plates has not been entirely successful. Tightening the bolts to the required torque may damage the glass coating. The metal components corrode, causing unsightly rust streaks on the tank exterior and may contaminate the tank contents.
Washers formed from elastomeric materials, such as are described by Fukushima et al. in U.S. Pat. No. 5,193,960 have been developed to reduce the localized stresses on the tank surfaces and to reduce or prevent leakage at the fastener. While helpful, these washers do not prevent the corrosion of the nut and the resulting streaking and contamination.
With tanks containing liquids or gasses, the bolt hole must be sealed against leakage. Similarly, where a dry powdered or granular material is stored, external liquids, such as rain, must be kept out of the storage structure. A variety of sealants or adhesives have been used. These tend to complicate installation, making threading a nut onto the bolt difficult. With cap-type nuts which are desirable to cover the exposed bolt end both from the point of view of leakage, bolt corrosion and appearance, excess sealant may prevent the nut from being fully tightened.
Installing the nut, washer and bolt assembly at each of often many thousand locations in a large structure is very time consuming. The need to hold a bolt in position in a hole, place a washer over the extended bolt end, then start the nut thread and tighten the nut to the required torque takes often results in installation problems. The washer may fall off the often short exposed bolt end and have to be retrieved and replaced, the nut may not start on the treads easily or may become cross-threaded.
Thus, there is a continuing need for bolt, washer and nut type fasteners for large structures, such as tanks, that can be easily, quickly and accurately installed, seal against leakage, reduce or eliminate corrosion and provide a pleasing appearance.